1. Field of the Invention
The present invention relates to a method of and an arrangement for limiting the level of a video signal present in digital form.
2. Description of the Related Art
Ever since digital integrated circuits became available which can be operated at sufficiently high clock rates, the digitization of video-signal processing has played a significant part in the design television receivers and video-cassette recorders (VCRs). As described in "Electronik-Sonderheft," No. 57, 1984, pages 97-100, in video-cassette recorders, the luminance information of the originally amplitude-modulated composite color signal to be recorded on magnetic tape is converted into a frequency-modulated signal in order to take advantage of the higher noise immunity of frequency-modulated signals. In connection with the frequency modulation, preemphasis is applied, i.e., the high frequencies of the video signals are emphasized before the signal reaches the frequency modulator. Luminance steps which occur in a television picture on sudden changes from black to white, e.g., in the presentation of sharp contours of objects, contain high frequency components. During preemphasis, these would result in correspondingly high instantaneous frequencies of the frequency-modulated signal. Recording such high frequencies on magnetic tape is problematic. In addition, the frequency modulator would be overloaded by such high-frequency signal components.
To avoid these problems, signal limiting is necessary before the preemphasis process.
In the reproducing portion of the video recorder, deemphasis is applied which counteracts the preemphasis used in the recording portion in a complementary fashion to achieve as perfect a reproduction of the recorded video information as possible. In the deemphasis process, high frequency components are correspondingly lowered.
The limiting before the preemphasis process falsifies the video information to be recorded. As a result, a sudden change from black to white, for example, will be reproduced on the screen not "clean", but only in a low-contrast, blurred form.